ABSTRACT
Extensive studies on cavitation have been conducted for years as an undesirable phenomenon in marine engineering. The cavitation phenomenon causes severe damage to the body of torpedoes, ship's propellers, pump impellers, and turbines. It also increases the frictional drag of submarine vessels and imposes many limitations on their design and construction. With increasing speed in submarines and increasing cavitation, supercavitation occurs which can be used to create a large bubble around the vessel. Because drag in the water is mostly due to the friction of the body, by the supercavitation, the contact of water with the torpedo is reduced. As a result, the friction drag is greatly reduced and the torpedo speed will increase significantly. In this regard, by artificially injecting air into the body in the state of cavitation, the process of creating supercavitation and reaching this stage can be accelerated. In this study, to investigate the effect of torpedo nose shape on supercavitation, different types of torpedo bodies have been modeled and studied different nose shapes. After modeling, meshing is performed with ANSYS Fluent software and the analysis of the model in cavitation numbers (0.2, 0.3, and 0.4) will be examined. Finally, the model will be analyzed in air injection mode (artificial supercavitation) with ANSYS CFX software and the results will be presented.

INTRODUCTION

Countless numerical analyses have been performed in fluid flow analysis around various objects. In the analysis of a hydrofoil near the free surface (Shariati and Mousavizadegan, 2017) or the investigation of the interaction between the fluid and the structure on the semi-submerged platform body (Bakhtiari et al., 2019) and the investigation of the effect of the hydrodynamic parameters on the hydrodynamic coefficients of the semi-submerged propeller (Donyavizade et al., 2018) is among these activities. Among the subsurface weapons, the supercavitation torpedo has unique features such as high speed and destructive power, which has given great importance to this powerful weapon. The most prominent issue for underwater torpedoes, as a very powerful weapon, is to reach high speeds in the shortest possible time, and this issue is very important and must be considered in the design and construction of such weapons. Be noted Today, the fastest torpedoes have limited speeds of less than 40 meters per second due to the tension and friction caused by the contact of seawater with its shell. As we know, when an object moves in water, a layer of water molecules adheres to the body of the object and is pulled along with it. It is called a shell.

MATERIALS AND METHODS

 In this research, the simulation of the supercavitation flow on the model torpedo with different noses is first discussed. In this modeling, the homogeneous fluid approach based on the transmission equation model is used. Continuity and momentum equations for mixture and transfer equations for volume fraction are solved. The volume fraction equation has spring terms to determine the rate of mass transfer between the vapor and liquid phase, which can be determined with different mass transfer models. In this research, three model nozzles in the form of conical, conical, and disk cones with the same sequences are analyzed and investigated. For this, two different cavitation models implemented with Fluent software are used. The obtained results are compared and evaluated with the laboratory results and semi-experimental relations, which have an acceptable convergence.

RESULTS

 In this part of the current research, the effect of five different nose modes of the model torpedo on the cavitation length, which is created in different cavitation numbers (0.2, 0.3, 0.4, and 0.5), is investigated.
The moving speed of the torpedo is 40 meters per second and the maximum floating depth is 30 meters below sea level (if the depth is defined higher than this value, due to the speed, it is not possible to create natural cavitation), which due to the change of the cavitation number, the effect of the shape of the nose It can be checked on cavitation.

DISCUSSION AND CONCLUSION

 In this research, based on the finite volume numerical method, the Navier-Stokes equations and the steam transfer equation in terms of mass fraction were modeled and solved for model torpedoes with different noses in Ensys software, and the effects of changing the shape of the nose on the formation of cavitation around the object were investigated.
REFERENCES:
Bakhtiari, M., Shemshaki, S. and Sadri Nasab, M. 2019. Investigating the interaction of fluid and structure on the body of a semi-submerged platform under the influence of regular sea waves. Journal of Marine Science and Technology, 18(1), pp. 62-73. doi: 10.22113/jmst.2018.99592.2053.
Donyavizadeh, N., Shafaghat, R., Dardel, M. and Mohammadzade Negharchi, S., 2018. Investigation of effect weight of parameters affecting the hydrodynamic coefficients in design of surface-piercing propellers.
Khalil Shariati, S. and Hossein Mousavizadegan, S., 2017. Infinite length hydrofoil analysis near the free surface.
Reichardt, H., 1945. The physical laws governing the cavitation bubbles produced behind solids of revolution in a fluid flow. The Kaiser Wilhelm Institute for Hydrodynamic Research, Gottingen, Rep. UM, 6628.